1. Technical Field
This disclosure relates generally to thin-film magnetoresistive read sensors and particularly to the use of biasing layers to stabilize magnetic freelayers in configurations that implement “two dimensional magnetic recording”.
2. Description
Magnetic recording heads can bias the reader freelayer by means of the fringe field of the so-called bias layers. This approach is taught by Yanagisawa et al. (U.S. Pat. No. 8,462,467 B2). The bias layer itself is usually a soft magnetic material, and needs to be stabilized by means of magnetic coupling, usually exchange coupling, to another 2nd magnetic layer which is itself stabilized by some means. Usually this 2nd magnetic layer is the upper shield (just above the bias layer) and is itself stabilized by an antiferromagnetic layer such as IrMn, as is taught by Garfunkel et al. (U.S. Pat. No. 8,514,524 B2).
It is instructive to examine the approach of Yanagasawa a bit more closely by examination of an illustration of the structure taught by him in schematic FIG. 1. Referring to schematic FIG. 1, there is shown a longitudinally biased sensor arrangement. The patterned multilayered sensor structure (layers not shown in detail) is labeled 20. An insulating layer, 70, electrically isolates the sensor from symmetrically placed laterally disposed biasing layers, each labeled 60. An upper shield is 50, and a lower shield is 40. The two layers, 60, that are the biasing layer elements, are always soft magnetic films. A stabilizing or unidirectional anisotropy (shown by arrows in 50 and 60) is provided by shield 50 to biasing layers 60 and the shield anisotropy is itself pinned by exchange coupling to the top layer C, labeled 30, which is usually an antiferromagnetic (AFM) layer of IrMn.
Referring next to FIG. 2, there is shown another prior art scheme, this one with multilayer biasing. Note that biasing layer, bracketed as 60, is now formed as a synthetic antiferromagnetic (SyAFM) laminate of two antiferromagnetically coupled layers 62 and 66, (their oppositely directed magnetic anisotropies shown as arrows), separated by an exchange coupling layer 64. But the biasing layers are always soft magnetic material which may be coupled by an exchange coupling layer typically Ru, at positions 64 and 80. The exchange coupling layer 80 provides a coupling to upper shield 50. Whether a single soft magnetic layer is used for biasing as in FIG. 1, or a coupled lamination as in FIG. 2, unidirectional stabilization is ultimately provided by AFM layer 30, also labeled C.
Schemes such as this may work well when there are no geometry constraints and there is room for the antiferromagnetic film, C. However, for so called Two Dimensional Magnetic Recording (TDMR) designs, which employ two sensor structures formed one over the other, Garfunkel et al. (U.S. Pat. No. 8,824,106 B1) have shown that it is critical to minimize the film thicknesses and, correspondingly, the distance between the lower and upper sensors. There is a need to reduce this sensor-to-sensor distance while also providing improvements to the biasing.